Vehicle restraints are used in the material handling industry to prevent vehicles from moving away from a loading dock while the vehicle is being loaded and/or unloaded with goods or materials. In general, these devices act as substitutes for wheel chocks. But unlike wheel chocks, conventional vehicle restraints typically engage the Rear Impact Guard (“RIG”) bar of the vehicle. RIG bars (which can also be referred to as “ICC” bars) are horizontal members that extend across the rear end of the vehicle. In the U.S., regulations require that the vertical distance between the bottom edge of the RIG bar and the ground not exceed 22 inches at any point across the full width of the member, and that the rearmost surface of the RIG bar be within 12 inches of the rear extremity of the vehicle.
Trailers and other transport vehicles tend to “float” up and down as they are loaded and/or unloaded at loading docks. More specifically, as weight is moved off and on the vehicle it moves up and down, respectively, thereby varying the vertical position of the RIG bar relative to the ground. Some restraint systems have been developed to accommodate this vehicle movement, and they generally fall into three categories. The first category employs a restraining member operably coupled to a carriage having rollers or similar devices which ride on tracks mounted to the face of the loading dock. See, for example, the vehicle restraints disclosed in U.S. Pat. Nos. 4,472,099, 4,443,150, 4,282,621, 4,264,259 and 4,695,216, each of which is incorporated herein by reference in its entirety. The use of a vertically moving carriage provides a range of motion to engage RIG bars at different heights. However, the carriage rollers are subjected to vehicle restraint loads while moving up and down in response to vehicle loading and unloading. As a result, this type of restraint generally requires relatively high maintenance to service the moving carriage and related parts. Additionally, some of these vehicle restraints are designed to operate in response to vehicle impact. More specifically, to operate the restraint the vehicle backs into the loading dock until the RIG contacts the restraint system, causing the restraint system to move a locking hook into engagement with the RIG bar. The repeated shock of the RIG bar on such systems can cause significant component wear. Additionally, because the carriage track is mounted to the dock face, in some situations it may interfere with operation of the dock leveler, particularly on relatively low loading docks.
A second category of restraint system includes a vertical bar or similar restraining member that is moved into position in front of the RIG bar to prevent forward movement of the vehicle away from the loading dock. Various types of mechanisms have been proposed to position the bar in such systems, such as those disclosed in, for example, U.S. Pat. Nos. 4,634,334, 4,605,353, and 4,784,567, each of which is incorporated herein by reference in its entirety. In particular, some of these restraint systems pivot the bar into the vertical position to restrain the vehicle. One shortcoming of this type of system, however, is that the raised height of the bar is constant and, as a result, it may interfere with hitches and/or other equipment mounted to the underside of the vehicle.
A third category of restraint system utilizes one or more hooks which pivot about a fixed hinge mounted to the dock wall. See, for example, U.S. Pat. Nos. 4,605,353, 4,208,161 and 4,605,353, each of which is incorporated herein by reference in its entirety. In this type of system, the distance from the dock wall to the hook varies as the hook moves through its arc of travel to engage the RIG bar, and as the vehicle moves up and down during the loading/unloading process. If the final distance between the hook and the dock face after the loading/unloading process is less than the distance when the process started, the RIG bar may impart such a high load on the hook that the hook may not release when desired.
All of the restraint systems described above operate by restricting horizontal movement of the transport vehicle away from the loading dock. This movement may be caused by a variety of factors, such as the driver inadvertently attempting to drive away from the loading dock while the restraint is engaged, the slope of the ground, and/or the kinetic energy imparted to the vehicle by the loading and unloading of goods and materials. Of these, the most common causes of vehicle horizontal movement are the accelerations/decelerations imparted to the vehicle by loading and unloading of goods and materials by hand, fork lift, etc.
Regardless of the cause of the movement, if the vehicle has moved away from the loading dock at the conclusion of the loading/unloading process, it can put a load on the restraining member of the restraint system, whether the restraining member is a blocking member, a rotating hook, etc. Although this situation is not unsafe, it can lead to an operational issue referred to as “hook pinch.” Hook pinch occurs with vehicle restraint systems when the restraining member is loaded by the transport vehicle to the extent that, when the dock operator attempts to disengage the restraining member from the RIG bar and return the restraint system to the stored position, the operator is unable to do so because of binding between the restraining member and the RIG bar caused by the vehicle load. More specifically, in such situations the restraint system is not powerful enough to overcome the binding force and disengage the restraining member from the RIG bar. Typically, the only way to relieve this force so that the restraining member can be disengaged is to have the vehicle driver move the transport vehicle a slight distance back against the dock bumpers and away from the restraining member. This operation is called “bump-back,” and can be a time-consuming effort in that it requires coordination between the dock operator and the transport vehicle driver. Accordingly, it would be advantageous to provide an improved vehicle restraint system that addresses the problem of hook pinch.